The production of a high voltage for an x-ray tube may use inverter circuits operated at resonance. Such resonant converters are described, for example, in U.S. Patent Application Publication No. 2008/0198634 A1.
In a resonant inverter having a series resonant circuit that includes a series capacitor and a series inductor, and half-bridge actuation or full-bridge actuation, the output voltage and the output power may be set by varying the actuation frequency of the semiconductor switches in the bridge paths. If a transformer is used in the inverter for the purpose of DC-isolation or for setting up the voltage, the stray inductance of the transformer may perform the function of the series inductor, and only a series capacitor may be used. Depending on whether the actuation frequency is below or above the resonant frequency of the resonant circuit, a distinction may be drawn between sub-resonant and super-resonant actuation.
For sub-resonant actuation, the actuation frequency may be very low for low output powers, and may reach the audible frequency range. A multi-resonant inverter may be used to overcome this disadvantage (e.g., by connecting an inductor in parallel with the capacitor of the series resonant circuit). An output power of zero is achieved just for an actuation frequency close to the parallel resonant frequency, such that the frequency range of the actuation may be limited to a sufficiently narrow band. This approach may be used for the design of the output-side smoothing capacitors and the EMC filters.
However, in the case of sub-resonant actuation, the switching-on of a switch involves the reverse-connected diode in parallel with the opposite switch of the bridge path being commutated off. As a result, high switching losses (e.g., at relatively high switching frequencies) may occur. To reduce the switching losses, additional passive or active snubber networks may be used that allow zero-current switching. The current in the series resonant circuit is commutated from the switch to the diode with only small switching losses arising.
For super-resonant actuation, the voltage transformation and the power transmission close to the resonant frequency are at a maximum, and the actuation frequency may be greatly increased for relatively low powers. The resonant frequency may be chosen such that the actuation frequency is always above the audible range. Due to the parasitic capacitances and inductances of the transformer that cause additional points of resonance, it may be impossible to restrict the range of the actuation frequency, similar to sub-resonant actuation. The resonant circuit may be designed such that the resonant frequency is below the maximum output voltage for the minimum input voltage and the minimum actuation frequency desired for the maximum output power.
For example, when an additional range is to be covered for the input and output voltages, an inconvenient ratio for reactive and active power in the inverter away from this operating point may result. As a result, excessively high conductance losses are produced. For super-resonant operation, the reverse-connected parallel diodes may not be commutated off, but the current in the series resonant circuit may be actively disconnected by the switch, thereby producing switching losses. However, the switch-off losses may be minimized by capacitive switching load relief in the form of capacitors that are connected in parallel with the switches. At the moment of switching, zero-voltage switching is obtained.